The term “nanoparticle” generally refers to particles that have an average diameter between about 1 nm to 100 nm. Nanoparticles have an intermediate size between individual atoms and macroscopic bulk solids. Nanoparticles typically have a size on the order of the Bohr exciton radius, or the de Broglie wavelength, of the material, which allows individual nanoparticles to trap individual or discrete numbers of charge carriers, either electrons or holes, or excitons, within the particle. The spatial confinement of electrons (or holes) by nanoparticles is believed to alter the physical, optical, electronic, catalytic, optoelectronic and magnetic properties of the material. The alteration of the physical properties of a nanoparticle due to confinement of electrons is generally referred to as quantum confinement effects.
Nanoparticles may exhibit a number of unique electronic, magnetic, catalytic, physical, optoelectronic and optical properties due to quantum confinement effects. For example, many nanoparticles exhibit photoluminescence effects that are significantly greater than the photoluminescence effects of macroscopic molecules having the same composition. Additionally, these quantum confinement effects may vary as the size of the nanoparticle is varied. For example, size-dependent discrete optical and electronic transitions exist for clusters of Group II-VI semiconductors (e.g., CdSe) or Group III-V semiconductors (e.g., InAs).
This loss of energy level degeneracy, however, has not previously been observed in the optical properties of Group IV nanoparticles (e.g., silicon (Si) nanocrystals). In Si, for example, the lowest lying energetic transition violates conservation of momentum; therefore, light absorption requires phonon assistance (a phonon is a quanta of vibrational energy), resulting in a very low transition probability. Consequently, bulk Si photoluminescence is very weak. Quantum confinement in Si nanocrystals and porous Si, however, leads to enhanced luminescence efficiencies with quantum yields that have reached as high as 5% at room temperature and blue-shifted “band gap” energies. However, in sharp contrast to their direct band gap semiconductor counterparts, Si nanocrystals have not displayed discrete electronic transitions in the absorbance and photoluminescence excitation (PLE) spectra.
The wet chemical techniques used to synthesize Group II-VI and III-V semiconductors have not been readily applied to Group IV materials, largely due to the high temperatures required to degrade the necessary precursors. Typically the temperature required to degrade the necessary Group IV precursors exceeds the boiling points of typical solvents. Furthermore, the strong covalent bonding of Si requires temperatures higher than the Group II-VI materials to achieve highly crystalline cores. Moderate progress has been made with alternative solution-phase reduction of Si salts and aerosol methods. These methods, however, have produced nanocrystals with extremely broad size distributions. Aerosol methods have required a thick oxide coating to stabilize their structure, which has been shown recently to significantly affect the photoluminescence (PL) energies of porous Si.